During the folding with a folding press, deflections occur inside the two frames, these deflections being the more important as the dimensions of the press are important. These deflections depend, in particular, on the shape, the dimensions and the quality of the material to fold, and they are generally not identic for both frames. In particular, when folding a piece, the upper frame has a tendency to bend upwards under the effect of the thrust of the working cylinder pistons, generally arranged at each of the ends of the press, leading to a longitudinal upwards deflection of the surface of the upper die holder. Similarly, the lower frame has a tendency to bend downwards leading to a downwards longitudinal deflection of the surface of the lower die holder. As a result, the respective working surfaces of the two frames and, therefore, of the dies are no longer parallel. The depth of penetration of the male die into the female die will then no longer be the same over the whole length of the press. It follows that the folding angle obtained is not the same over the whole length of the piece which is folded.
In order to remedy these difficulties, certain folding presses comprise compensation cylinders placed in one or both of the frames and arranged to compensate for the deflections due to the working of the press and to obtain, in particular, similar deflections of the upper and lower frames. However, these systems require a great number of trials for adjustment and most of them require the user to determine in advance the deflection due to the operation of the press.